Hi Mundell
That is indeed a mystery

I cannot find any trace of Helen Kenley among the Waltons. Could it possibly been a mistake on John's part when giving his mother's name on his marriage certificate. I know from experience that could be true. On my maternal grandmother's death certificate her maiden name was completely wrong, it should have been Dickson but was given as Forsyth. It was fortunate that I DID know for sure that her maiden name was indeed Dickson. One of her sisters was married to a Forsyth, so it could be human error on your John's part. Intriguing though that he gave your grandmother the middle name of Kenley.
I don't know if you have these but on trying to find an answer to the mystery I found 3 other children born to John Walton and Elizabeth Rollo (there could be more) -
Robert Walton 1839 Kemback
James Walton 1840 Cameron
Elizabeth Dickson Walton 1851 Carnbee.
It appears from this that Elizabeth Rollo was still alive in 1851 at least, 9 years before the marriage of John in1860.
Have you tried checking the census for 1851 onward? I looked up the 1841 census and John and Elizabeth are residing at Radernie, Cameron, with their children Robert aged 2 and James aged 6 months.
I also found siblings for your Alison (you probably know of these already but just in case) -
Robert 1860 Scoonie (probably died in infancy as younger brother also named Robert)
Andrew 1862 "
Robert 1863 " middle name Bisset
John 1863 " (again, probably died in infancy as younger brother also named John)
John 1865 "
Janet 1868 Kirkcaldy
James 1870 Strathmiglo
then of course your Alison b. 1972 back in Scoonie.
I hope you get to the bottom of this mystery, as they say always more questions than answers!
Regards from Jenc.